Two technically challenging problems in crewed spacecraft and submarine life support are carbon dioxide (CO2) separation and concentration, and safe oxygen (O2) generation by water electrolysis. To add to the complexity, the processes of CO2 separation and concentration and safe O2 generation must be microgravity compatible for spacecraft use.
Recovery of O2 in CO2 as water by reduction with hydrogen (H2) requires concentrating CO2 from its low partial pressure in air to one atmosphere, a concentration factor of more than a hundred. This is done thermophysically on the International Space Station (ISS) with complex, heavy, energy-intensive equipment.
Liquid water electrolysis on the ISS is accomplished using an electrolysis cell that includes a cation-exchange membrane, typically a copolymer of polytetrafluoroethylene (PTFE) and polyfluorosulfonic acid (PFSA), such as DuPont's Nafion®. The ISS uses a single Nafion® cation-exchange membrane electrolysis cell. The ISS electrolysis cell system requires a very pure feed of water, high-speed rotary inertial gas-liquid separators, and an explosion-proof enclosure. The explosion-proof enclosure is necessitated by safety concerns that are inherently present in single membrane electrolysis cell systems. In such systems, the O2 and H2 gases are separated by only the single membrane. A failure of the membrane would allow the O2 and H2 gasses to combine, leading to a possibility of explosion. Systems that can tolerate only a single failure before the possibility of explosion arises are referred to herein as one-fault-tolerant. Explosion-proof enclosures are required for safety when using one-fault-tolerant water electrolysis systems. The explosion-proof enclosures protect human life and protect the integrity of the hull of the vehicle (or other structure) within which the one-fault-tolerant system (e.g., the single Nafion® cation-exchange membrane electrolysis cell of the ISS) operates. In order to eliminate the need for explosion-proof containers for water electrolysis systems, it would be desirable to make use of a water electrolysis system that would require two or more components to fail (i.e., multi-fault-tolerant) before the possibility of explosion would occur.
Because of the explosion potential inherent in a one-fault-tolerant water electrolysis system, and the consequent need for explosion-proof enclosures, systems of the kind described above are limited in size and weight. That is, the equipment itself must be small enough to fit inside of the explosion-proof enclosure. In volumetrically limited environments, such as the ISS, a submarine, a bunker, or a tank, room that could have been used for life support system equipment is reduced by the room occupied by its explosion-proof enclosure. It is desirable, therefore, to eliminate the need for an explosion-proof container in order to allow for additional room for life support equipment (or other equipment).
Moreover, known systems, such as that in the ISS require multiple machines with multiple moving parts, to perform the task of air revitalization. It is desirable, therefore, to reduce the number of machines used, the complexity of the machines, and the number of moving parts used in the overall performance of air revitalization task. Reduction of the number of machines saves space, while reduction in the number of moving parts increases overall reliability of the system due at least to failure of a moving part.
Furthermore, weight is very often a concern in the environments in which a water electrolysis system could be used. An explosion-proof enclosure, robust enough to contain an explosion of the kind described, will, of necessity, be heavy. It would therefore be beneficial to have a water electrolysis system that does not require an explosion-proof enclosure, in order to reduce the weight associated with the life support equipment.
Still further, in environments that have limited access to the outside world, the quantity of spare components carried in anticipation of a component failure is an important factor to be evaluated in the selection of a life support system. Therefore, it is desirable to have a life support system that incorporates a plurality of the same devices. In this way, only one spare part is needed to replace any one of the plurality of same devices.
Additionally, energy storage and generation are often limited in environments that make use of life support systems. Accordingly, it is desirable to have a life support system that utilizes less energy, while providing the same or more life support functionality, than known systems.